Scanning aperture partially coherent optical correlator

ABSTRACT

A partially coherent optical correlator of, for example, the frequency plane type, has a slit of adjustable width that moves rapidly in the spatial domain P1 to scan a portion of the input signal recorded on a film strip which is moving relatively slowly in this same domain. The slit produces a plurality of correlation samples that are added incoherently in an energy detector located in the output plane P3 to give one sample of the partially coherent correlation function for each scanning cycle. Repeated scanning cycles develop the full, partially coherent, correlation function.

United States Patent [1 1 Williams Oct. 30, 1973 SCANNING APERTUREPARTIALLY COHERENT OPTICAL CORRELATOR Primary Examiner-David Schonberg[75] Inventor: Ross E. Williams, Yonkers, NY. Assistant Emmmerf McGrawAttorney-R. S. Scrascta et al. [73] Assignee: The United States ofAmerica as represented by the Secretary of the Navy, Washington, D.C.[57] ABSTRACT A partially coherent optical correlator of, for example,[22] Filed July 1972 the frequency plane type, has a slit of adjustablewidth [21] Ap 1. No.: 272,537 that moves ra id] in the s atial domain Pto scan a P P y P portion of .the input signal recorded on a film stripwhich is' moving relatively slowly in this same domain. g bzig za gifgwg The slit produces a plurality 'of correlation samples [58] Fie'ld356/7l 165 166 that are added incoherently in an energy detector lo-350/162 cated in the output plane P to give one sample of the partiallycoherent correlation function for each scan- [56] References Cited ningcycle. Repeated scanning cycles develop the full,

UNITED STATES PATENTS partially coherent, correlation function.2,787,l88 4 1957 Berger 356/167 4 Claims 2 Dmwing Figures DRIVE SCANNINGDRIVE FILM PHOTOMULTIPLIER COLLIMATOR POINT LIGHT SOURCE MOVING SLlT APERTURE SIGNAL FUNCTION PLUS NOISE REFERENCE FUNCTION TRANSPARENCYPatented Oct. 30, 1973 2 Sheet s-Shet 1 Patented ()c t. 30, 1973 2Sheets-Sheet n TRANSPARENCY MOTION (SLOW) IN PI INCOHERENTLY SUMMEDCORRELATION FUNCTION OVER ALL COHERENCE INTERVALS RELATIVE DISPLACEMENTBETWEEN SIGNAL AND REFERENCE FUNCTIONS Fig. 2

' strip, illuminated normally by monochromatic, coherent, collimatedlight, is advanced in the object plane P of a simple spherical lens. Asis well known, a Fourier transform relationexists between the lightamplitude distributions at the'front and back focal planes of such alens. Consequently, a reference transparency having the complexconjugate of the Fourier transform of the above signal function recordedthereon, is inserted in the back focal plane of the spherical lens. Thisplane P is the first frequency plane of the correlator. A secondspherical lens, which has the reference transparency in its front focalplane, performs an inverse Fourier transform, and a suitable energydetector is positioned in the back focal plane P of this second lens.

The plane P in which the signal function is introduced, is called thespatial domain, while the plane P in which the reference transparency isintroduced, is called the frequency domain. As an alternative toinserting the reference transparency in the frequency domain, atransparency with an appropriate transmittance characteristic may beutilized in a subsequent spatial domain of the correlator.

In the case where the signal that is being processed in the correlatoris partially distorted along its time base so that it does not have acoherent relationship with the signal recorded, those separate portionsof the correlation function which appear in the output plane P; willhave dissimilar phase relationships. Hence, destructive interferenceoccurs when their amplitudes are directly added by the photomultipliertube, which is a typical energy detector commonly used in thecorrelator. This interference effect thus limits the usefulness of thestandard correlator in processing slightly distorted or partiallycoherent signals, for with these signals there is no appreciable outputsignal from the correlator and, consequently, the correlator does notindicate the fact that there may be some degree of coherence between theinput signal and the reference signal.

In underwater object detecting and locating systems and in geophysicalexploration systems, the coherence of the received signal with respectto the radiated signal is limited by, for example, perturbations of thesignal source and receiver motions, the movement of reflecting objects,and by unpredictable movements taking place within the water massitself. All 'of these conditions, however, result in signal changes thatare relatively slow compared to the signal duration. Thus, the receivedsignal may be considered a partially coherent one.

According to the present invention, the film strip which carries theinput signal is moved relatively slowly across the aperture of anoptical correlator of the type generally described above. However, asthis movement takes place, a slit, whose width may be adjusted, is movedrapidly in the spatial domain P, to scan a portion of the input signaltrack. This scanning can be back and forth or only in the direction offilm travel. This movement of the slit produces a plurality ofcorrelation samples that are added incoherently in the energy detectorin the output plane P to give one sample of the partially coherentcorrelation function for each scanning cycle. Repeated scanning cyclesdevelop the full partially coherent correlation function as the signalfilm moves through plane P By repeatedly scanning the relatively slowmoving film strip at an appropriate speed, the slit, in effect, samplesthe correlation functions from individual segments of the signal, allfor a given displacement of the signal relative to the referencefunction, and causes these correlation samples to appear in rapidsequence at the photomultiplier tube or energy detector where they areadded incoherently. Because the slit examines only a relatively shortsegment of the input signal, and because the width of the correlationfunction is inversely proportional to the bandwidth of the signal-beingprocessed, the peaks of the individual correlation functions so producedare sufficiently broad, especially in the case of FM signals, tooverlap. The photodetector thus can sum up these individual functions,and the resultant is a partially coherent correlation of the distortedinput signal and the replica, for example, represented by the recordedinformation on the transparency in the frequency domain It isaccordingly a primary object of the present invention to provide anoptical correlator which can be utilized with partially distortedsignals.

Another object of the present invention is to provide a standard spaceor frequency plane optical correlator with a scanning aperture ofadjustable width for processing signals which are partially coherent.

Another object of the present invention is to provide an opticalcorrelator wherein selected segments of a distorted signal may becompared with the reference function.

Other objects, advantages and novel features of the invention willbecome apparent from the following detailed description of the inventionwhen considered in conjunction with the accompanying drawings wherein:

FIG. 1 schematically illustrates one simplified embodiment of thepresent invention; and

FIG. 2 shows a plurality of the correlation function envelopes whichoccur at the output plane, 'together with the resultant obtained whenthese individual envelopes are added incoherently.

Referring now to FIG. 1 of the drawings, it will be seen that thepartially coherent optical correlator of the present invention in itsschematic form includes a point light source 10, a collimator 1 1, apair of spaced spherical lenses 12 and 13, and an energy detector 14,all symmetrically disposed with respect to an optical axis 15. A signalwhich is to be correlated and which may, perhaps, be mixed with noise isrecorded on a photographic film strip 16 which is arranged to beadvanced in the front focal plane P of lens 12 in a direction transverseto the above optical axis. Any suitable drive mechanism with anappropriate speed control feature, such as 17, may be used for thispurpose.

The input signal, as noted hereinbefore, is recorded as a densityvariation on film strip 16 and, depending upon the particular dataprocessing application involved, one or more signal tracks may existacross the width of the film strip.

Positioned in the back focal plane P of lens 12, the transform plane, isthe reference transparency 18. This transparency lies in a planetransverse to the optical axis of the system so as to be parallel 'tothat portion of the film strip 16 which is being processed. Recorded onthis transparency is the complex conjugate of the Fourier transform ofthe input signal. Thus, for example, if the correlator is being used inthe receiving portion of an underwater object detecting system toprocess reflected or echo signals, the reference transparency would haverecorded thereon the complex conjugate of the Fourier transform of thesignal initially radiated into the fluid medium. A second spherical lens13 is positioned such that reference transparency l8 lies in its frontfocal plane and a photomultiplier tube 14 or any other suitable energydetector, such as a second photo graphic sheet, is positioned in theback focal plane of this lens. This lens performs the inverse Fouriertransform.

The apparatus just described, it will be recognized, constitutesa'standard frequency plane optical correlator. In this type ofcorrelator, the input signal is introduced in a space plane, and thereference signal is inserted in the frequency plane. The correlationpeak appears off of the optical axis 15 in the output plane P and movesopposite to the direction of film strip advance, tracking the movinginput signal, appearing on axis only when the input signal and thereference function are aligned. Alternatively, as noted hereinbefore theinput signal and the reference function both may be inserted in spaceplanes. This is the so-called space plane correlator-and, in thisarrangement, the peak does not move in the manner-described above butoccurs on the optic axis in the output plane only at that time when thetwo functions are aligned. Thus, returning to the frequency planecorrelator of FIG. 1, if the signal recorded on film strip 16 isundistorted, then the correlator peak first appears off of the opticalaxis 15 and is undetected by photomultiplier tube 14. As the film isadvanced, this peak, as noted above, progresses to an on-axis pointwhere it is registered by photomultiplier tube 14, thus indicatingcorrelation between the compared signals. This correlation function hasa half width which is inversely proportional to the bandwidth of theinput-signal being processed.

If the signal present in the space plane P is distorted along its timebase, that is, if the signal is a swept FM signal and portions thereofdo not have the correct frequency because of a Doppler shift, forexample, then the individualcorrelation amplitude samples derived fromthe various different portions of this signal will have dissimilarphases as they appear in the output plane P Consequently, there will bedestructive interference when they are added by energy detector 14. Theoutput of the system, therefore, will be reduced to almost a zero level.and, thus,.the system will not provide an indication of the extent ofthe correlation which may exist between this distorted signal and thereplica registered in the reference transparency.

According to the present invention, a scanning slit whose width may bevaried is moved in the space plane 1 by scanning drive means 19 rapidlypast a segment of film strip 16, the length of which normallycorresponds to the full aperture of the correlator. This movement may beback and forth over this length of the strip or slit 20 may be cut in acircular plate such that the rotation of this plate causes the slit tomove past the film strip in one direction through a certain arc of itstravel. The particular setting of this slit, that is, its width alongthe input signal track, is adjusted by any appropriate overlying shuttermeans, such as 21, until maximum signal is observed at the output ofphotomultiplier 14. The slit should scan the full signal length indetermining proper slit width. In principle, it could be done at oneposition along the signal film, but since the signal time base usuallyis distorted in a somewhat irregular manner an appropriate slit width tomatch the average distortion along the signal time base should bechosen.

With the slit added to the correlator, only that portion of the inputsignal which is exposed by the slit is processed, with the referencefunctions stored in the transform plane P fThus, "a plurality ofcorrelation functions are developed in the output plane I, as differentsegments of the input signal track are exposed to the referencefunction. The energies of these individual correlation functions aredetected by energy detector 14 in the usual manner.

Because the slit reduces the length of input signal being processed to amuch smaller segment than that normally treated in the full aperturesystem, the amount of distortion present within the slit is appreciablyreduced. Thus, a peak correlation amplitude nearly equal to that for afully coherent signal can be realized in plane P using this smallersegment of the input signal. The individual correlation amplitudesdeveloped in plane P;, from the separate segments of the input signalstill have the phase dissimilarities mentioned above, but since theseare now developed sequentially in time the energy detector sums onlytheir energies and not their complex amplitudes. Hence, destructiveinterference is avoided. However, for an FM signal the individualcorrelation functions do not possess the narrowness of the correlationfunction associated with the full aperture system. This means that ifthe input signal is fully coherent the correlator output with theslit'is not as sharp or definitive as that which would be produced inthe standard system. However, because the individual correlationfunctions are broader, they can be incoherently added with appreciableoverlap by the energy detector, and this permits the present system toproduce an effective output when the input signal is distorted along itstime base.

The operation of the scanning slit may better be appreciatedfrom thefollowing description. As the slit moves to the right, as viewed in FIG.1, along the direction of film advance, at its first point in time itwill process a corresponding finite portion of the distorted inputsignal and, as shown in FIG. 2, a sample 1 of the correlation function30 appears in output plane P As the scanning slit moves across thesignal film, samples 2 3,, 4,, 9, are produced and added incoherently inthe energy detector to produce the partially coherent correlation sampleN The slower motion of the signal film through the aperture of plane P,causes a second set of samples, 1,, 2 3,, 9,, to be realized and addedincoherently to give'N, on the next scan of the slit across the signalfilm. Thus, the partially coherent correlation function samples N N .N,are produced on successive scans of the'slit as the signal film movesslowly through its aperture. Note that the peaks 31, 33, 35, 37 of thecorrelation functions 30, 32, 34, 36 for the individual segments of thesignal film are not aligned in the X direction so that they all occur onthe same scan of the slit. They occur, instead, at slightly offset,off-axis locations, indicating the presence of a degree of distortion inthe time base of the input signal.

Because the scanning slit moves fast compared to film strip 16, itproduces a sequence of correlation samples from the different segmentsof the input signal, all for a given average relative displacement, ormisalignment, in the X-direction between the signal and referencefunctions. The individual segments of the signal have different actualdisplacements relative to the reference function because of signal timebase distortion, as shown in the misalignment of the correlation peaks31, 33, 35, 37 in FIG. 2. As the signal film moves slowly, thedisplacement relative tothe reference function is changed, and the fullshape of the partially coherent correlation function 45 is developedslowly. The amplitudes of the correlation samples from individualsegments of the signal are added incoherently in the energy detectorbecause the latter squares the amplitude incident upon it at any onetime. The motion of the scanning slit causes the amplitudes 1,, 2 3 9 tobe incident at the energy detector at different times and therefore theyare combined to give the incoherent sum N, only after squaring. Itshould be appreciated that the incoherent sum of these samples from theindividual signal intervals illustrated results in a partially coherentcorrelation with the width of the main peak broader than the usualcorrelation function but similar to the individual functions from whichit is derived.

It should be appreciated that the individual correlations depicted inFIG. 2, such as 30, 32, 34, etc., are simplified to the point where onlythe envelopes are shown. These envelopes modulate a carrier frequencywhose phases are not identical from one interval to the next because ofthe time base distortions that are present. However, since theindividual amplitude samples due to the operation of the scanning slitare squared before summing in the energy detector, the phaseinconsistencies that would otherwise have reduced the coherent amplitudesum of wave form 45 to a very small value are removed. The squaredamplitudes, that is, the intensities, are all positive and, therefore,they may be added without any cancellations due to any phase mismatch.

It would be pointed out that the actual width of the scanning slit isdetermined by trial and error if nothing is known ahead of time aboutthe signal coherence of the input signal. This involves merely adjustingthe slit so as to obtain maximum output from the photomultiplier. Thisadjustment can be made as the slit is scanned for a few cycles, thephotomultiplier output observed, and then appropriate changes made tomaximize this reading.

Once this setting is determined, only small changes need be made usuallyto accommodate subsequent gradual changes in coherence. These gradualchanges, as indicated hereinbefore, which may be due to oceanconditions, receiver motions, etc., occur rather slowly and, thus, thereis no difficulty in tracking the coherence of the input signal.

The method of the present invention is applicable to any arbitrarysignal design, but it works particularly well with swept frequency FMsignals, a chirp signal commonly utilized. The reason for this is thatthe Fourier transforms in plane P of different segments of the signal inspatial plane P are nonoverlapping and therefore do not produceinterfering noise backgrounds for each other. Additionally, thebandwidth of a short segment of a swept PM signal is small. Therefore,the width of the main lobe of the correlation function is wide and, whenseveral main lobes are added together in the incoherent energy additionin output plane P they will overlap easily even though the signaldistortions in plane P, may displace the centers of the main lobes inoutput plane P What is claimed is:

1. In an optical correlator for processing an intput signal distortedalong its time base which is recorded as a density variation on a filmstrip moving in a first spatial domain of said correlator, theimprovement of a scanning slit of adjustable width positioned adja centsaid film strip and repeatedly scanning a portion of this film strip ata speed high as compared to the speed 7 at which this film strip movesin said domain,

each scanning cycle producing at the energy detector of said correlatorone sample of the partially coherent correlation function and repeatedscanning cycles developing the full, partially coherent, correlationfunction.

2. An optical correlator for processing partially coherent signalscomprising, in combination,

a first spherical lens;

a recording strip having input signal information stored therein asdensity variations in the transparency of said strip positioned in theobject plane of said lens;

a light source located on the optical axis of said lens for illuminatingan area on said strip with monochromatic coherent light;

means for advancing said strip across the optical axis of said lens suchthat different areas of said strip are illuminated by said light source;

a reference strip having the complex conjugate of the Fourier transformof a particular signal recorded thereon as density variations in thetransparency of this strip positioned in the backfocal plane of saidlens;

an energy detector;

a second spherical lens located on saidoptical axis, said referencestrip being in the front focal plane of said second lens, and saidenergy detector being in the back focal plane of said second lens;

a scanning slit positioned adjacent said recording strip and having anopening as measured along said strip of variable size; and

means for repeatedly moving said slit past a portion of said recordingstrip at a speed higher than the speed of advancement of said recordingstrip.

3. In an arrangement as defined in claim 2 wherein said energy detectoris a photomultiplier tube positioned on said optical axis and whereinthe setting of said slit corresponds to the opening size thereof whichdevelops a maximum signal in the output of said photomultiplier tubefrom the movement across said optical axis of a particular portion ofsaid recording strip having input signals stored therein.

4. In an arrangement as defined in claim 2 wherein the speed at whichsaid slit moves past said recording strip is such that a multiplicity ofscanning cycles occurs in the time a particular portion of saidrecording strip takes to move across said optical axis.

1. In an optical correlator for processing an intput signal distorted along its time base which is recorded as a density variation on a film strip moving in a first spatial domain of said correlator, the improvement of a scanning slit of adjustable width positioned adjacent said film strip and repeatedly scanning a portion of this film strip at a speed high as compared to the speed at which this film strip moves in said domain, each scanning cycle producing at the energy detector of said correlator one sample of the partially coherent correlaTion function and repeated scanning cycles developing the full, partially coherent, correlation function.
 2. An optical correlator for processing partially coherent signals comprising, in combination, a first spherical lens; a recording strip having input signal information stored therein as density variations in the transparency of said strip positioned in the object plane of said lens; a light source located on the optical axis of said lens for illuminating an area on said strip with monochromatic coherent light; means for advancing said strip across the optical axis of said lens such that different areas of said strip are illuminated by said light source; a reference strip having the complex conjugate of the Fourier transform of a particular signal recorded thereon as density variations in the transparency of this strip positioned in the back focal plane of said lens; an energy detector; a second spherical lens located on said optical axis, said reference strip being in the front focal plane of said second lens, and said energy detector being in the back focal plane of said second lens; a scanning slit positioned adjacent said recording strip and having an opening as measured along said strip of variable size; and means for repeatedly moving said slit past a portion of said recording strip at a speed higher than the speed of advancement of said recording strip.
 3. In an arrangement as defined in claim 2 wherein said energy detector is a photomultiplier tube positioned on said optical axis and wherein the setting of said slit corresponds to the opening size thereof which develops a maximum signal in the output of said photomultiplier tube from the movement across said optical axis of a particular portion of said recording strip having input signals stored therein.
 4. In an arrangement as defined in claim 2 wherein the speed at which said slit moves past said recording strip is such that a multiplicity of scanning cycles occurs in the time a particular portion of said recording strip takes to move across said optical axis. 